Estimation of Active-Fault Shape Using Radon Concentration in Soil Gas and Numerical Simulation

摘要

Radon prospecting using the #alpha#-scintillation counter method has been executed in theFutagawa fault area, situated in the southeasternKumamoto plain, to clarify geometrical features of thefault. The Futagawa fault has proven to be active bythe aerophotograph interpretations and fieldinvestigations. Along six lines perpendicular to thefault, 602 points are set at 2.5-rn intervals to pumpup soil gas from the borehole of 65-cm depth. A theoryfor calculating number of radon in the soil gas, whichconsiders the gas condition with respect to theradioactive equilibrium, is applied to evaluate thetrue ~222Rn concentation at each measurement point. Theobtained concentrations are higher than those observedin the areas without active faults. It should be notedthat several points on the fault system are found tohave young soil gas under the nonequilibrium condition.Two characteristics of the radon concentrations can bepointed out as the follows: the first is goodcorrelation between the concentrations and thelocations of lineaments, and the second is asymmetricshape of the concentration anomalies. Based on anumerical simulation using the finite differencemethod for the diffusion and movement of radon, shapemodels of the Futagawa fault are constructed by fittingthe calculated radon concentrations to the measuredones and, consequently, the fault is interpreted asnormal fault type dipping northward with the range from55deg to 76deg. The origin of the radon in the studyarea is also discussed using the radioactivemeasurement datasets of the samples taken from theborehole cores that penetrate the fault.